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Dalton Transactions
the carbene complexes shows increased electron density at the
carbene carbon of 18 compared to 17 (0.095 vs. 0.174 respect-
ively, Table 3), meaning that the tethered thioether is capable
of modulating the electron density at the distal carbene. This
remote modulation of electron density could account for the
reactivity and also be a factor in reducing the barrier for the
rate-limiting diazo extrusion step,67,68 thereby increasing cata-
lyst turnover. Theoretical calculations are underway to better
address the effects of tethered ligands on transition states and
energy barriers in diazo-mediated transformations.
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3. Conclusions
In conclusion, heteroleptic dirhodium(II) paddlewheel com-
plexes containing axially coordinated thioether ligands were
synthesized using a microwave-assisted approach. We have
demonstrated that catalyst synthesis and reactivity is influ-
enced by the tether length and the substituent of the
thioether. The tethered complexes achieved higher yields that
their non-tethered counterparts in the cyclopropanation of
styrene with ethyl diazoacetate, even demonstrating good
yields with equimolar amounts of styrene substrate. Electronic
studies and computational methods provided insight into the
effect of the donating strength of the tethered ligands on the
observed reactivity. The calculations were of particular rele-
vance, revealing the importance of the effect of the tethered
thioether on the HOMO of the RhII-carbene complexes. Efforts
to further understand the interplay between the donor of the
tether and reactivity in RhII-carbene mediated reactions are
currently ongoing.
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18 F. G. Adly, Catalysts, 2017, 7, 347.
Conflicts of interest
19 S. Miah, A. M. Z. Slawin, C. J. Moody, S. M. Sheehan,
J. P. Marino, M. A. Semones, A. Padwa and I. C. Richards,
Tetrahedron, 1996, 52, 2489–2514.
There are no conflicts to declare.
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24 R. S. Drago, J. R. Long and R. Cosmano, Inorg. Chem., 1981,
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Acknowledgements
We are grateful for financial support from the University of
Tennessee Knoxville in Knoxville, TN. We thank John F. Berry
(University of Wisconsin–Madison) for useful discussions con-
cerning the electronic structure calculations. We also thank
the personnel in the NMR, Biological and Small Molecule
Mass Spectrometry Core, and analytical facilities at the
University of Tennessee. We also wish to thank Prof. David
Jenkins for facilitating our electrochemical experiments.
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15786 | Dalton Trans., 2020, 49, 15779–15787
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